RONTGEN RAYS 315 



to the path of the Rontgen rays, the scattered secondary rays 

 preponderate in a direction at right angles to the original stream 

 of cathode rays. That the rays are found not to be completely 

 polarised can be ascribed to the fact, supported by other evidence, 

 that the cathode rays are not stopped in a single collision, but 

 describe random and irregular paths along many directions 

 before coming to rest. 



In many respects, besides that of speed, the pulses resemble 

 waves of visible light ; both consist of electric and magnetic 

 forces at right angles to each other and to the direction of 

 propagation, and both can exert a pressure on the medium 

 in the direction of propagation. This pressure in the case of 

 light is the pressure of radiation which has been measured by 

 Lebedew, Nicols and Hull and Poynting: in the case of the 

 X and 7 rays the effect may be traced on the distribution of 

 the secondary electrons produced when matter is encountered. 

 The ionisation produced by the rays represents the effect of the 

 electric force in the wave front, in causing the ejection of 

 electrons out of encountered atoms. 



The ionising effect of ultra-violet light on gases has been 

 demonstrated by Lenard (1900), and more recently by J. J. 

 Thomson (1907) : the much more marked effect due to Rontgen 

 and 7 rays is, of course, well known. We are thus introduced 

 to a resemblance between the rays and light in which the effects 

 differ only in degree. 



Now, even in the most favourable circumstances, only an 

 exceedingly small fraction— less than one in a billion — of the 

 molecules of a substance subjected to the Rontgen rays becomes 

 ionised. If all the molecules are equally exposed to the rays, as 

 would be implied by the above form of the pulse theory with 

 its continuous wave front, those which are ionised must be in 

 a state very far removed from the average. The abnormal 

 condition of such molecules cannot be attributed to the posses- 

 sion of an exceptional amount of kinetic energy, for the kinetic 

 theory of gases would then require that the ionisation in a 

 gas (if its molecules are subject to the Maxwell-Boltzman 

 law of distribution) should vary rapidly with the temperature : 

 this is entirely opposed to present experiment, which indicates 

 that the ionisation is independent of the temperature. 



The most plausible of the other assumptions that are open to 

 us, if we are to retain the continuous pulse, is that the ionisation 



